“PIZZAGATE,” AS the social media conspiracy lovers styled it, is the deranged conspiracy theory by which Comet Ping Pong, a popular family pizzeria in Northwest Washington, is the hub of a child-abuse ring run by Hillary Clinton and her campaign chairman, John Podesta. This baseless drivel is fake news at its most patently absurd.

But what is absurd can yield events that are all too real, as on Sunday when a North Carolina man armed with an assault rifle reportedly barged into Comet and fired at least one shot. The man, Edgar Maddison Welch, told police he’d gone to the restaurant to “self-investigate” pizzagate, which took hold in the dark crevices of social media in the days before the election.

It’s tempting to think of fake news as a political dirty trick devised by partisan mischief-makers that caters to ignoramuses in election season. But the Comet Ping Pong incident is a reminder that fake news isn’t only a distortion of public events, which is bad enough. Fervid conspiracy theorists have seized on anodyne personal emails from the hacked Podesta files to conjure, out of the thinnest air, supposed evidence of a child sex-trafficking ring, with innocent plans for social gatherings transformed into alleged opportunities for pedophilia and names and pictures of innocent parties posted on the Internet. Public and private malice are melded.

The crackpots who peddle such stuff include Michael G. Flynn, the son and top aide to retired Lt. Gen. Michael T. Flynn, who is President-elect Donald Trump’s pick to be the new White House national security adviser. Both Flynns are especially fond of conspiracy theories — the elder one spread similar pedophile nonsense about Ms. Clinton, and the younger one called on Comet Ping Pong to disprove a negative by dispelling the pedophile bunk. They are thereby complicit in fouling the public discourse and in the incitement that led to Sunday’s attack.

In the case of Comet Ping Pong, the infection has spread down the block, literally, to neighboring shops and businesses that have also received threats for their supposed involvement in child abuse — for no other reason than their proximity to Comet Ping Pong, which itself is guilty of nothing more than occasionally serving a limp pizza.

Mr. Trump’s embrace of conspiracy theories created a safe zone for fact-free discourse. His birther obsession; his hint that Justice Antonin Scalia may have been murdered; his assertions that Muslims in New Jersey cheered as they watched the Sept. 11 attacks; his comments associating Sen. Ted Cruz’s father with President Kennedy’s assassination — all of that has normalized the bogus and dissolved the opprobrium that traditionally attached to public figures who traded in falsehoods.

Beyond the responsibility that social media platforms have shirked in allowing their spaces to be polluted by fiction masquerading as fact, there is little to prevent the spread of fake news. In Comet Ping Pong’s case, though, two things might help: D.C. police should post a uniformed officer at the restaurant. And both Mr. Flynns, along with Mr. Trump, should visit Comet Ping Pong and order one of its excellent pizzas. We recommend the Jimmy, topped with meatballs.